The present invention relates to an automatic focusing device that is employed in, for example, a photographic camera.
In recent years, a camera equipped with an AF (automatic-focus) function is on the increase, and on single lens reflex cameras with interchangeable lens as well, the AF function has become indispensable. In general, in single lens reflex cameras, a so-called phase difference detecting method is adopted for automatic focusing. The AF with the phase difference detecting method is executed with steps such as the following:
Firstly, a pair of object images with a spatial parallax are projected, respectively, on a pair of photosensitive units, such as a CCD (charge coupled device), etc., and the light amount received by the respective photosensitive unit is integrated in terms of time. Then, according to the phase differential of two object images on respective photosensitive units, the distance differential between the sensing element (film equivalent plane) and the imaging plane (focus position) of the photographing lens with respect to a photographing object, and the direction thereof (defocus amount/defocus direction) are calculated.
From the calculated defocus amount and direction, drive amount of a motor necessary to drive the photographing lens to make the imaging plane coincident with the film equivalent plane is obtained, based upon which the focus lens is driven along the optical axis thereof. The number of pulses applied to the motor in the above operation is obtained according to the following formula: EQU P=Kv.times.D
Where,
P is the number of pulses applied to the motor, PA1 D is the defocus amount, and PA1 Kv is a so-called lens-movement-amount conversion coefficient (K value) which is a coefficient representing the relation between the defocus amount and the number of pulses to drive the motor as necessary to make the defocus amount zero, and is the value inherent to the respective lens. PA1 a focus lens that is movable along an optical axis thereof; PA1 a drive means for driving the focus lens; PA1 a distance measuring means for obtaining a defocus amount of the focus lens with respect to a photographing object; PA1 a computing means for computing a relative speed of movement of the photographing object with respect to the focus lens along the optical axis, based upon defocus amounts obtained by the distance measuring means; PA1 a drive control means for controlling the drive means to drive the focus lens, based upon results of a computation by the computing means, to a position where an in-focus condition is obtainable with respect to the photographing object after an elapse of a certain time; PA1 means for determining whether the position of the focus lens falls into a predetermined focus allowance; and PA1 an operation control means for repeatedly executing the distance measurements, the computations and the driving of said focus lens, regardless of whether it is determined that the position of the focus lens falls into the predetermined focus allowance.
FIGS. 30 through 32 explain a conventional AF system, as above described. In each drawing, "object image position" indicates the imaging plane of the photographing lens with respect to the photographing object with the position of the focus lens taken as a reference, and "focusing position" is the film equivalent plane, also with the position of the focus lens taken as a reference.
In FIG. 30, as a result of a distance measurement executed at time 0, assume that the distance differential between the focusing position and the object image position, i.e., the defocus amount is detected as D0. Then, to make the defocus amount DO zero (0), the lens is driven. When the photographing object is stationary or standing-still, the focusing position becomes consistent with the object image position by the results of the driving of the lens. Under this state, interrupting processing of release ON is executed, and an exposure starts after elapse of a release-time-lag t2 which is, the time required for mechanical operations for mirror ascent and stopping down of aperture. During exposure, as illustrated in FIG. 30, the focusing position and the object image position remain consistent with each other.
However, when the object is moving (more particularly, moving in the lens drive direction), even if integration and computation are once carried out during its movement, as the object keeps moving while the lens is being driven according to the results of such integration and computation, further integrations, computations and resulting lens drives must be repeatedly executed to keep the focusing position and the object image position consistent.
FIG. 31 shows the case wherein a photographing object is moving from a remote field to a near field at constant speed. The amount of movement of the object image position becomes larger as the object is the closer to photographing lens.
Assume that distance differential between the object image position and the focusing position, i.e., the defocus amount, at point 1 is D1. When the lens is driven by an amount corresponding to D1 and after elapse of time t1, defocus amount D2 is obtained at point 2. In the same manner, the lens is driven for the amount corresponding to D2, and after the elapse of time T2, defocus amount D3 is obtained at point 3. Here, the focusing position at point 2 corresponds to the object image position at point 1, and, since the object keeps moving while time T1 elapses, the defocus amounts would be: EQU D1&lt;D2&lt;D3.
Thus, the defocus amount gradually increases each time when the distance measurement is executed, while the object is moving towards the photographing lens at a constant speed. Therefore, the lens drive can not sufficiently follow the movement of the object image position.
In order to overcome the problem, the above delay should be prevented by predicting the amount of movement of the object image position from the start of integration to the completion of the computed lens drive, with which the lens is additionally driven.
However, even if the lens is driven to the predicted in-focus, position, the lens drive is stopped at that position and a further lens movement is not executed until the object moves out of a predetermined focus allowance after the lens position falls into the focus allowance, although the object is continuously moving even after the additional lens drive is completed. This means that a well-focused photograph may not be taken during the time where the object moves out of the focus allowance to the completion of the further lens drive for the next in-focus shot. Particularly, when the object approaches the camera, the time required to drive the lens to the predicted in-focus position increases, which makes it difficult to take a well-focused photograph.